I can make a solid argument for the hockey set being a very close runner-up for most nuts Pro Set issue. Over the last ten years, it has supplanted 1990 FB as my favorite to open, specifically because there are still new finds being revealed in recent years, while 1990 FB "feels" mostly tapped out.
Because Pro Set believed in a "living set" philosophy, they stopped/started and changed print runs throughout the entirely season. This led to tons of recurring print flaws, minor and major variations (some that could only be found in one specific printing) that most other companies' products didn't have to such a degree. The very low cost of this stuff makes chasing this stuff very appealing to me.
Then there are the what-ifs and the mythology surrounding the set: what happened to the announced Final Update set and who would've been on the checklist? Who is the goalie on the series 2 box (mock-up card)? Did pro set issue any of the traded snipe players without their snipe as they repeatedly stated to hobby media and in their press releases? If so, where are they?
Unlike 1990 Pro Set FB, the hobby hasn't seen any big discoveries of dumped corporate/staff stashes. One guy found a bunch of major variation uncut sheets (Manley, Marion) in pile of trash set out on garbage day, complete with hand written post-its attached to them, maybe 15 years ago. I believe there is a lot more out there but we may not see much of it until a larger portion of the generation holding on to it dies off and/or starts to dump it.
Owner and creator of JunkWaxGems, online source for unlisted error, variation and oddball info from the junk wax era.